1. Field of the Invention: This invention relates to a binder composition for injection molding. More particularly, it relates to a binder composition for injection molding which is excellent in various properties such as capability of molding, degreasing and sintering.
2. Description of Prior Art:
Metal powders such as iron or alloy powders and ceramic powders are generally injection molded by a process comprising the steps of adding a binder to a metal or ceramic powder to thereby give a composition, injection molding the composition with an injection molding machine, removing the binder from the molded product, i.e., degreasing, and sintering the molded product thus degreased to thereby give the aimed product.
The binder to be used in the above process should satisfy the following requirements.
(1) The composition containing the binder can be injection molded with an injection molding machine.
(2) The injection-molded product obtained from the above composition has such a strength as not to be broken until entering the subsequent sintering step.
(3) The molded product can be readily degreased, i.e., it can be degreased within a short period of time at a low degreasing temperature.
(4) The degreased product has a sufficient strength.
(5) The molded product has a high sinterability. That is, it shows a small dimensional change and a high strength and can give a highly dense product.
As such a binder, thermoplastic resins and waxes have been employed (cf. Japanese Patent Laid-Open No. 121150/1984 and No. 121151/1984).
Although these known binders satisfy the requirement (1) as described above, other properties thereof are still unsatisfactory. For example, a conventional binder mainly comprising a wax or a thermoplastic resin requires at least several tens of hours for degreasing. Such a prolonged degreasing can yield nothing but a small molded product. Further the resulting product frequently suffers from blistering, cracking or breakage during the degreasing step. Thus an excellent product can be scarcely obtained thereby. When no thermoplastic resin is employed, on the other hand, the obtained composition shows a poor moldability, which brings about a serious problem in practice.